Method of preparing metal coated metallic substrates



United States Patent 3,503,775 METHOD OF PREPARING METAL COATED METALLICSUBSTRATES Lowell W. Austin, Weirton, W. Va., assignor to National SteelCorporation, a corporation of Delaware No Drawing. Filed Apr. 12, 1966,Ser. No. 545,788 Int. Cl. B05b 5/02; B44d 1/092, N44

US. Cl. 117-17 18 Claims ABSTRACT OF THE DISCLOSURE This inventionrelates to a novel method of eleetrostatically depositing finely dividedcoating metal on metallic substrates, and to a method of compacting andheat treating or fusing the resulting particulate metal coatings. Theinvention further relates to the metal coated products thus produced.

Finely divided nonmetallic coating materials which are not goodconductors of electricity have been successfully electrostaticallydeposited heretofore on a variety of substrates. For instance, a mixtureof powdered glue of the remoistenable type and a thermoplastic binderhas been electrostatically deposited upon paper in producing the gummedpaper products of commerce, and refractory oxide annealing separatorshave been electrostatically deposited on metallic sheet or strip.

When preparing the above mentioned products, it was not necessary toprecoat the surface of the substrate with a binder for the purposes ofcausing the particles of coating material to adhere and assuring thatthe resulting particulate coating was uniform and had adequate greenstrength. However, when an effort was made to electrostatically depositan electrically conductive coating metal under prior art conditions onthe surface of an electrically conductive metallic substrate, theresulting particulate metal coating was not uniform and did not havesufficient green strength to allow handling prior to compacting and heattreating or fusing.

The inherently low green strength characteristic of the prior artmetallic coatings electrostatically deposited on metallic substrates hasbeen overcome heretofore by precoating the surface with a tacky binder.The tacky binder serves to anchor the finely divided metallic particleson the surface of the substrate and to thereby improve the greenstrength of the particulate coating sufficiently to allow handling priorto compacting. However, it is neces sary to remove the binder prior tocompacting the metal particles and this is a very difiicult task. Atleast traces of the binder tend to remain on the substrate surface or onthe surfaces of the metal particles and the residual binder contaminatesthe compacted metal coating.

It has also been proposed to increase the green strength of particulatecoatings electrostatically deposited on a dry uncoated substrate byusing very finely divided metal particles having a mean mass particlesize not greater than 15 microns, and preferably not greater than about5 microns. However, while this method is very effective for producingthin metal coatings and has met with surprising technical success, thepresent cost of the extremely finely divided metal powders that arerequired is too high for an economic commercial process which must becom- 3,503,775 Patented Mar. 31, 1970 ice petitive with other coatingmethods such as electrodeposition. Also, it is difficult to produce suchfinely divided metal powders in the large quantities required for highvolume commercial production.

It has been discovered unexpectedly that the surface of a metallicsubstrate may be coated with a thin film of water, and thereafterparticles of a coating metal may be electrostatically deposited upon theresulting surface, which is free of an oily or tacky binder, in the formof a uniform particulate metal coating which has sufiicient greenstrength to enable the coated substrate to be handled. It has beenfurther discovered that low cost relatively coarse metal particles whichare commercially available in large quantities may be electrostaticallydeposited on metallic substrates in the absence of a tacky binder, andin the form of heavy or light coatings of a controlled desiredthickness. Surprisingly, it has been still further discovered that thewater film does not adversely affect the substrate nor the coating metalparticles in any way, and that the water film may be completely removedwithout any difficulty. Thus, for the first time it is not necessary toapply a tacky foreign material which is difiicult to remove from thesurface of the substrate, or to use high cost extremely finely dividedmetal powders.

It is an object of the present invention to provide a novel method ofelectrostatically depositing a finely divided coating metal on ametallic substrate having a surface which is substantially free of atacky binder, in the form of a particulate metal coating Which hassulficient green strength to allow handling prior to compacting.

It is a further object to provide a novel method of compacting and heattreating or fusing the particulate metal coatings of the invention toform a substantially continuous coherent and adherent layer of thecoating metal on the substrate.

It is still a further object to provide the improved metal coatedmetallic substrates produced in accordance with the invention.

Still other objects and advantages of the invention will be apparent tothose skilled in the art upon reference to the following detaileddescription and the examples.

In practicing the present invention, a film of water is applied to thesurface of a metallic substrate, and thereafter a particulate coating ofa coating metal is electrostatically deposited by introducing a drygaseous suspension of electrically charged particles of the coatingmetal adjacent to the surface of the substrate while it is positioned inan electrostatic field which directs the charged particles to thesubstrate.

The substrate surface should be uniformly coated with a thin film ofliquid water at the time of introducing the gaseous suspension of metalparticles and electrostatically depositing the particulate coating. TheWater should be present in an amount sufficient to form a substantiallycontinuous film of liquid over the surface area to be coated with themetal particles, and in an amount insuflicient to form pools or streamsof liquid which collect on the surface and cause uneven or nonuniformcoating. The substrate surface should be free of oil, grease and othersubstances which are not wetted by water in order to form a more uniformfilm. In accordance with one preferred procedure, the substrate surfaceis washed free of surface contaminants, the cleaned surface is rinsedwith excess fresh water, and then the excess water is removed and a thinfilm of liquid water is metered onto the surface by passing thesubstrate between rubber wringer rolls. The water film also may beapplied to a substrate surface which is dry initially by contacting itwith wet brushes, wet sponges or the like, or by spraying on acontrolled amount of water. In some instances, better results may beobtained if the water used in coating the substrate contains a small buteffective amount of a prior art surfactant or wetting agent to reducethe surface tension of the water and assure more uniform coating of thesurface.

The particles of the coating metal are sufliciently small to be readilysuspended in a stream of air or inert gas and deposited on the surfaceof the substrate in the form of a particulate metal coating havingsuflicient temporary adhesion and cohesion to allow handling prior tocompacting or fusing.

For better results, the particles of the coating metal should have amean mass particle size not greater than about 50 microns, andpreferably not greater than about 30 microns, in the maximum dimension.The lower limit on the particle size is largely practical in nature, andparticles having a mean mass particle size of 0.5-1 micron or smallermay be used. Metal particles having a mean mass particle size notgreater than about 20 microns, such as about 20 microns, produce aparticulate coating which is very uniform and which has excellentadhesion and cohesion in the green state. Metal particles having a meanmass particle size greater than microns and not more than about 30microns, such as -25 microns, produce good results and are often lowerin cost. Regardless of the particle size selected, it is possible toproduce more uniform and/or heavier particulate coatings of a desiredcontrolled thickness in accordance with the method of the invention thanwhen using a metallic substrate which is not coated with a film ofwater. Also, metal particles having a much larger particle size may beused and still obtain adequate green strength in the particulatecoating.

A wide variety of coating metals are satisfactory, pro vided they are inthe form of small particles within the range set out herein. Examples ofmetals include alumi num, antimony, cadmium, chromium, cobalt, copper,gold, iridium, iron, lead, magnesium, manganese, molybdenum, nickel,niobium, osmium, palladium, platinum, tantalum, tin, titanium, tungsten,vanadium, zinc and zirconium. Additionally, alloys including one or moreof the foregoing metals may be used. The alloys may include brass,bronze, stainless steel, Monel, high chromium ferrous alloys in generalsuch as an alloy containing 70% chromium and 30% iron, zinc-iron alloyssuch as an alloy containing 70% zinc and 30% iron, and aluminummanganesealloys, such as an alloy containing 10-70% manganese and the remainderaluminum together with incidental impurities. An alloy coating may beformed in situ on the substrate by applying a mixture of two or more ofthe metal powders in amounts to form the final alloy, preferablyfollowed by heat treating and compacting and heat treating or fusing, orthe alloy per se may be preformed, subdivided to the desired particlesize, and applied to the substrate to form an alloy coating inparticulate form, followed by compacting and heat treating or fusing.

Presently preferred coating metals include aluminum, alloys of aluminumsuch as the aluminum-manganese alloy mentioned above, copper, nickel,stainless steel, chromium and chromium-iron alloys, zinc and zincironalloys. Aluminum coatings are especially preferred for some purposes,such as in the manufacture of container stock.

The metal particles may be substantially spheroidal to irregular inshape, such as particles having the appearance of spheres or grains ofsand under a microscope, or in the form of finely divided plates. Thespheroidal or irregularly shaped particles may be formed by grinding thedesired metal in massive form, atomization of molten metal, or by otherwell known methods. The plate-like particles may be formed bydisintegrating thin metal foil, or by rolling spheroidal or irregularlyshaped particles.

The substrate may be any suitable metal, but it is preferably steel orother ferrous metal. Steel sheet or strip of indefinite length isusually preferred for commercial operations, as it is possible to carryout the invention in a continuous or substantially continuous manner.

The substrate is in an electrostatic field which is applied in a mannerto drive the charge particles of coating metal to the surface of thesubstrate. The substrate may have an electrical charge on its surfacewhich is preferably opposite in polarity to that of the particles of thecoating metal at the time of introducing the gaseous suspension thereofadjacent the surface. The specific manner in which the gaseoussuspension of electrostatically charged particles is produced, as wellas the specific manner in which the electrostatic field is produced, arenot of importance to the present invention. Any suitable prior artapparatus and method may be employed, including those disclosed in US.Patent No. 3,090,353.

In operating one type of apparatus, a metallic substrate which may beferrous metal strip having a thin continuous film of liquid Water on itssurface is passed throughan electrostatic deposition zone in acontinuous manner. A rapidly flowing stream of air, inert gas, reducinggas, or other suitable gaseous medium containing particles of thecoating metal is passed into the electrostatic deposition zone andbetween the substrate and a plurality of electrodes in the form of wiresor sharp points spaced from the strip and extending transversely to thedirection of movement of the strip. The strip is electrically grounded,and a high voltage positive or negative potential is applied to theelectrodes. A corona discharge having the same polarity as the appliedhigh voltage potential is thereby caused to take place about theelectrodes. The gas surrounding the electrodes is ionized, and thespacing of the electrodes is such that in the region between the highvoltage electrode and the strip there is a predominance of electricallycharged ions of one polarity. The dispersed particles of the coatingmetal become electrically charged by ion bombardment, and then theelectrostatic field acts upon the charged particles to propel themtoward the strip. The particles of the coating metal are deposited onthe strip surface in the form of a uniform particulate metal coating.The particles lose their electrical charges upon contact with the stripsurface, and the particles are then held on the strip surface bymolecular or dispersion forces sometimes called Van der Waals forces.

The green particulate metal coating has sufiicient temporary adhesionand cohesion to allow handling. However, it should be treated to form acontinuous layer of the coating metal which is permanently coherent andadherent to provide a satisfactory article of commerce.

One method of treating the green particulate coating includes a preheattreatment, compacting the heat treated coating by rolling under pressureto form a continuous layer of the coating metal, and then subjecting thecompacted metal coating to an elevated temperature at which a coherentand tightly adherent substantially continuous layer of the coating metalis produced. The exact time and temperature for use in the preheattreatment will vary depending upon the selected coating metal. It isonly necessary that the preheat treatment be conducted below the meltingpoint of the coating metal and at a sufficiently elevated temperatureand over a sufiiciently long period of time to cause the particles ofthe coating metal to agglomerate or cohere and form a weak bond with thesubstrate, and prevent the metal particles from becoming dislodged orsticking to the rolls during the rolling step. When aluminum is thecoating metal, a temperature of about 2001050 F. and preferably 400 600F. may be used for the preheat treatment and the heat treatment may takeplace in air. The period of preheat treatment may vary over wide ranges,such as from about 15 seconds to 30 minutes or longer, and preferablyabout 1-5 minutes at 500-600 F. Temperatures and times for the preheattreatment of aluminum-manganese or other aluminum alloys, zinc orzinc-iron alloys, and other nonrefractory metals having similarproperties :may be approximately the same as for aluminum. Metals havinga high melting point, e. nickel, chromium and stainless steel mayrequire nonoxidizing or reducing atmospheres and higher preheattreatment temperatures and longer periods of time, such as heating atabout 800- 1700 F. and preferably about l100-l300 F. over about 15seconds-2 minutes to 5-60 minutes. As a general rule, within the aboveranges, shorter periods of preheat treatment are preferred for thehigher temperatures, and longer periods at the lower temperatures.

The preheat treated coated substrate may be passed between pressurerolls and rolled under sufficient pressure to produce a substantiallycontinuous layer of the coating metal. The rolling step may be conductedat normal room temperature, or at an elevated temperature such as thetemperature of the preheat or postheat treatment. The rolling pressureis sufficient to compact the particulate metal coating and may be, forexample, 2-10 tons per inch of width of substrate. When desired, therolling step may be conducted under pressure conditions so that there issome reduction in the thickness, such as /2-5%. Also, rolling may beused so as to generate heat for the postheat treatment, as well as tocompact the metal coating and/ or cause some reduction in the thicknessof the substrate.

The rolled substrate may be subjected to an elevated temperature atwhich a coherent and highly adherent substantially continuous layer ofthe coating metal is produced. The temperature and time of postheattreatment will vary somewhat from metal to metal, but in general anelevated temperature and period of time are satisfactory which cause thelayer of the coating metal to adhere tightly to the substrate surface,and which also cause the metal particles to bond together and produce acoherent coating. When the coating metal is aluminum, a satisfactorypostheat treating temperature and period of heating is about 850-1050 F.for about 15 seconds or longer, and preferably about 900l000 F. over 1-5minutes, but a longer period of time may be used such as from 30 minutesto 2 hours. Postheat treating conditions similar to aluminum aresatisfactory for aluminum-manganese or other aluminum alloys, but oftena lower temperature such as 500-600" F. is useful for zinc and zincalloys. When nickel, chromium, stainless steel, or other coating metalshaving high melting points are used, then nonoxidizing or reducingatmospheres and higher postheat treatment temperatures are necessarysuch as about 800-l700 F. and preferably about l1001300 F., and theperiod of heat treatment may be about 15 seconds-30 minutes to 1-12hours or longer. As a general rule, the postheat treatment may beconducted at a temperature which is about two-thirds of that of themelting point of the metal, and the period of heat treatment is extendeduntil the desired degree of cohesion and adhesion is obtained.

Another preferred method of treating the particulate metal coating is byfusion of the metal particles to produce a coherent and adherentcontinuous metal coating. This method is especially effective when thecoating metal has a substantially lower melting point than the metalsubstrate, such as when a ferrous metal substrate is coated with tin,zinc or lead particles. The metal coating on the substrate may be merelyheated above its melting point to fuse the particles, and then theresulting molten coating is quenched to solidify the coating and producea metal substrate having a coherent and adherent substantiallycontinuous layer of the coating metal.

Irregularly shaped metal particles produce a brighter compacted coatingand are preferred where brightness in the compacted coating isdesirable. The metal particles should be dry and free of dirt, greaseand tacky binders, and need not be pretreated to improve adherence orcoherence, or the green strength of the particulate coating.

The metal particles are applied to the substrate in an amount to providea final compacted and heat treated or fused coating of a desiredthickness. For example, the final coating thickness may be 0.05-1.5 milsand preferably about 0.1-1 mil. It is also possible to apply more thanone metal coating to a given substrate. For instance, a first layer ofone of the coating metals disclosed herein may be applied to a ferrousmetal substrate, followed by application of a second layer which may bedifferent coating metal. The composite article thus produced may be usedas such, or it may be heat treated so as to cause the various layers ofcoating metals to diffuse and form an alloy layer. It is also possibleto apply an organic protective coating of paint, varnish, lacquer, orthe like over the compacted and heat treated or fused metal coatingsproduced in accordance with the invention.

It has also been discovered that the quality of aluminum coatings formedon ferrous metal substrates may be greatly improved by preheating thegreen particulate coatings at 400-600 F., and preferably by preheatingat 500600 F. in an elemental oxygen-containing atrnosphere such as airuntil a thin layer of a light yellow or straw colored oxide is formed onthe surface of the ferrous metal substrate. The temperautre of heatingshould not be sufficiently high to form an oxide coating having a bluishto black color, and the time of heating should not be extendedsubstantially past the time at which the water film is removed and avery thin light colored oxide layer is formed. Surprisingly, it has beenfound that preheating the green aluminum coatings at a temperature andtime effective to form such light colored oxides also greatly reduces oreven prevents picking or removal of the particulate coating by the rollsduring compacting by rolling, and aids in forming an adherent andcoherent substantially continuous layer of the coating metal. Ininstances where the rolled continuous aluminum coating tends to blister,the blistering may be controlled by rolling the agglomerated particulatecoating at an elevated temperature such as 400 F. or higher, andpreferably at about 400-600 F.

For best results, the temperature of the rolled continuous coatingshould not be raised rapidly during the final heat treatment. Usually,it is preferred that the rolled coating be conditioned by heat treatingat about 400-600 F. for a short period of time such as l-15 minutes, oruntil the occluded gases have been expelled and temperature equilibriumis reached, and then the substrate may be heat treated at a much highertemperature such as 800-1700" F. without damage to the substantiallycontinuous coating or loss of adherency,

Surprisingly, the film of liquid water does not adversely affect thesubstrate surface nor the coating metal particals which are depositedthereon. This is true even when the substrate is ferrous metal and thesubstrate is heat treated in air. The oxide which forms on a ferrousmetal surface when it is treated with water and/or heated to an elevatedtemperature in air usually adversely affects the bond between thecoating metal and the surface. Nevertheless, this does not occur whenpracticing the present invention. In fact, the oxide often has anadvantageous effect, such as when the water film is removed during apreheat treatment of a ferrous metal substrate at 400-600 F. prior torolling or compacting the green coating. The water film may becompletely removed during this preheat treatment and a residue of waterdoes not remain on the strip surface or in the final coating.

The present invention is especially useful for coating unannealed coldrolled ferrous metal substrates as the annealing step which is normallypracticed may be incorporated with the heat treatment steps of theinvention. For example, large coils of unannealed cold rolled steel maybe treated by a continuous prior art method to remove oil, grease andother contaminants from the moving steel surface, the cleaned steelsurface is washed with fresh water and a uniform thin film of liquidwater is continuously applied or metered onto the surface, andthereafter a desired coating metal is continuously electrostaticallydeposited on the moving wet, clean steel surface. The resulting greenparticulate coating may be given a preheat treatment as described hereinto agglomerate the metal particles, remove the water film and form athin light colored oxide, and is then compacted by passing the coatedsteel between pressure rolls where, if desired, a reduction in thesubstrate thickness may take place and a continuous layer of the coatingmetal is formed. The resulting unannealed, coated steel having acontinuous layer of the coating metal thereon is then annealed in areducing atmosphere following prior art practice, such as by boxannealing or by passing the coated steel in the form of strip through aprior art continuous annealing line. Thus, the coherency and adherencyof the coating layer may be improved and the steel substrate may beannealed in a single heat treatment. This combined coating and annealingprocess is especially useful in the application of coating metals havinghigh melting points to unannealed cold rolled steel strip, such asaluminum, chromium, manganese, molybdenum, nickel, titanium, vanadium,and alloys thereof including, for example, stainless steel and highchromium-iron alloys. High melting point coating metals usually requirea reducing atmosphere for the final heat treatment of the compactedcoating, and as a reducing atmosphere is present in the prior artannealing processes, the heat treatment and atmosphere of the annealingstep are performing two functions simultaneously and Without added cost.

Still further improved results may be obtained by application of thecoating metal by electrostatic deposition on a continuously movingsubstrate of indefinite length, such as ferrous metal strip, followed bycontinuous preheat treatment of the particulate coating immediatelythereafter, and continuous compacting of the heat treated metal coatingby passing the strip between pressure rolls. Preferably, the foregoingsteps are performed in line and without passing the strip around a guideroll to prevent any possibility of damage to the coating. Theresulting-coating is more uniform, and is more adherent and free fromdefects than when the steps are not performed continuously and insequence. The coating metal may be applied to one side of the substratesurface only or on both sides, as desired. This is of importance inpreparing certain products of commerce, such as ferrous metal sheetscoated on one side only with zinc, where the opposite or uncoated sideof the sheets is to be provided with a different type of protective ordecorative finish.

The foregoing detailed description and the following specific exampleare for purposes of illustration only, and are not intended as beinglimiting to the spirit or scope of the appended claims.

EXAMPLE i This example illustrates the present invention when using acontinuous electrostatic coating line for coating steel strip.

An annealed blackplate strip of tinplate gauge and quality iselectrolytically cleaned in an aqueous alkaline solution of known typefollowing prior art practices, rinsed in fresh water to remove thecleaning solution, pickled in aqueous sulfuric acid, rinsed in freshWater to remove the excess pickle liquor, dried and coiled.

The clean and pickled backplate strip is passed between a series offresh water sprays arranged to apply a small controlled amount of freshwater to the strip surfaces, and the strip is passed between rubberwringer rolls which uniformly distribute the water over the surface areaof the strip in the form of a thin, uniform film. The rubber wringerrolls also serve to remove any excess water. The resulting strip isimmediately passed through a horizontal electrostatic deposition zoneand a particulate metal coating is electrostatically deposited on thestrip surface in the presence of the film of water.

A plurality of charging wires are arranged in the electrostaticdeposition zone to extend across the width of the strip. The chargingwires are spaced four inches apart with respect to the direction of thestrip movement, and two inches above the strip surface. A total of eightstainless steel charging wires having a diameter of 0.005 inch are used.The ends of the charging wires are attached to end supports, and thewires are electrically connected to a high voltage source.

An air-blower and powder feeder assembly is used to generate a cloud orgaseous suspension of the metal powder for feeding to the electrostaticdeposition zone. The powder feeder is used to meter the metal powderinto the blower, and the powder is suspended in a stream of air and thegaseous suspension is blown through a conduit into the electrostaticdeposition zone. The terminal end of the conduit has a nozzle thereonprovided with a slotted outlet arranged to distribute the suspension ofpowder across the width of the strip, and to introduce the suspensionbetween the charging wires and the upper surface of the horizontallymoving strip. The nozzle is arranged to feed the suspensionlongitudinally along the strip and in the same direction as the strip ismoving. The strip is provided with an electrical ground. A negativepotential of 14-15 kilovolts is applied to the charging wires to produceions for charging the particles of the metal powder.

The speed of the blower is regulated to produce an air suspension fromthe metal powder fed thereto, and the air suspension is then introducedbetween the charging wires and the upper surface of the moving stripwhere the suspended metal particles are charged electrically. Thecharged metal particles are directed downward toward the moving stripsurface by the lines of force of the electrostatic field, and depositedthereon in the form of a uniform particulate metal coating. r

The coated strip emerging from the electrostatic deposition zone ispassed horizontally through an infrared oven and preheat treated asnoted in the table. The preheat treatment removes the water film byevaporation and agglomerates the metal particles sufliciently to causethe particles to cohere and form a weak bond with the strip surface. Theinfrared oven is provided with an air atmosphere, or a reducingatmosphere (H and N as noted in the table.

The strip having the agglomerated particulate metal coating on its uppersurface is passed horizontally to a to a rolling mill and immediatelyrolled under a pressure of approximately 4 tons per inch of width tocompact the particulate coating and effect a reduction of the strip ofabout 1-2%. The strip emerging from the rolling mill has a substantiallycontinuous uniform layer of the coating metal thereon, and it is thenpostheat treated at the temperature and time noted in the table toassure that a coherent and tightly adherent coating is produced. An airatmosphere or reducing atmosphere (H and N is maintained during thepostheat treatment, as noted in the table.

The metal coating on the resultant strip is tightly adherent as shown bythe Erichsen cup test. Also, the coating is substantially continuous andnonporous and provides good corrosion resistance when the coated side ofthe strip is subjected to corrosion tests.

A number of runs compared the various metal powders noted in the table.The metal powders have mean mass particle sizes varying between about 5microns and about 29 microns, and uniform coatings of the particulatemetal could be obtained in each instance which had sufiicient greenstrength to allow subsequent handling. However, when the water film isomitted, with all other conditions remaining the same, it is impossibleto produce uniform coatings having adequate green strength when usingpowders having a mean mass particle size greater than 15 microns. Thewater fihn also aids greatly in the electrostatic deposition of themetal powders having mean mass particle sizes less than 15 microns asthere is less shifting of the powder and the yield of the metal powderin the resulting coating is much greater. It is also possible to controlthe thickness of the applied coating more effectively, and heaviercoatings may be produced.

The metal powders and the treating conditions are given below in thetable. Very satisfactory coatings were 6. The method of claim 1 whereinthe coating metal comprises an alloy selected from the group consistingof brass, bronze, Monel, stainless steel, zinc-iron alloys,aluminum-manganese alloys and chromium-iron alloys.

7. The method of claim 1 wherein the coating metal is prepared in eachinstance. 5 aluminum, the substrate is heat treated at a temperature ofTABLE Preheat treatment Postheat treatment Particle size Temperature,Temperature, Metal (microns) F. Time (min.) Atmosphere F. Time (min)Atmosphere 5.3 500-000 1-3 oo-1,000 30 Air. 6.4 5004100 1-3 oo-1,000 30Air. 7.0 500-000 1-3 oo-1,000 30 Air. 13.9 500-000 1-3 900-1,000 30 Air.14.3 500-000 1-3 900-1,000 30 Air. 18.0 500-000 1.3 900-1,000 50 Air.20.0 500000 1-3 900-1,000 30 Air. 21.0 500-000 1-3 900-1,000 30 Air.25.0 500000 1-3 900-1,000 30 Air. 29.0 500-000 1-3 goo-1,000 30 Air.

9.0 1,300 1-2 1, 1-2 Hz-i-N: 13.5 1, 300 1-2 1, 000-1, 700 1 12 Hz+Nz30.0 1, 300 1-2 1, 000-1, 700 12 H2+NZ 13.0 1, 300 1-2 Hr+N2 1, 600-1,700 12 H2+N2 Hours.

What is claimed is: about 400-1050 F. to cause the aluminum particlesin 1. A method of coating a ferrous metal substrate with the particulatecoating to agglomerate, and the substrate acoating metal comprising thesteps of is heat treated at a temperature of about 400-1050 F. applyinga film of water on the surface area of the after compacting theagglomerated particulate coating to ferrous metal substrate to be coatedwith the coating produce a coherent and adherent substantiallycontinuous metal, the surface of the ferrous metal substrate belayer ofthe coating metal. ing free of an applied coating of a tacky binder and8. The method of claim 1 wherein the coating metal is the water beingpresent in an amount suflicient to selected from the group consisting ofnickel, chromium wet the surface area and form a thin substantially andstainless steel, the substrate is heat treated at a temif film of ter,perature of about 800-1700" F. in a nonoxidizing atmosintroducing agaseous suspension of dry electrically phere to cause the metalparticles in the particulate coatcharged particles of at least onecoating metal ading to agglomerate, and the substrate is heat treated atjacent to the surface of the ferrous metal substrate a temperature ofabout 800-1700 F. in a nonoxidizing having the film of water thereon,the particles of atmosphere after compacting the agglomeratedparticulate the coating metal having a mean mass particle size coatingto produce a coherent and adherent substantially not greater than about50 microns, continuous layer of the coating metal. the ferrous metalsubstrate being in an electrostatic 9. The method of claim 1 wherein theferrous metal field applied in a manner whereby the electricallysubstrate initially is cold reduced unannealed blackplate, chargedcoating metal particles are driven toward the and subsequent to rollingthe particulate metal coating the wet surface of the substrate and aredeposited th blackplate having the substantially continuous layer of onin the form of a particulate metal coating, coating metal thereon isheat treated at an elevated anheatihg the ferrous metal Substrate havingthe Water fi ncaling temperature to anneal the blackplate and produceand particulate metal coating thereon at an elevated a tightly adherentlayer of the coating metal. temperature to evaporate the Water film andProduCe 10. The method of claim 1 wherein the ferrous metal a driedparticulate metal coated subst ate, substrate is ferrous metal strip,the strip is passed conagglomerathlg the coating metal Particles ysubjecting tinuously through an electrostatic deposition zone and thethe dried particulate metal coated su s t an gparticulate metal coatingis electrostatically deposited slornerating temperature of at least butless thereon, the coated strip is passed through a heat treating thanthe melting Point of the coating metal until zone to agglomerate themetal particles, and the heat t Coating metal PertieleS haveagglomerated, treated strip having the agglomerated particulate metalthereafter Compacting the agglomerated Particulate coating thereon isrolled under pressure, the foregoing metal coating y rolling the CoatedSubstrate under steps of electrostatic deposition, heat treating toagglomerpressure to produce a substantial continuous layer ate the metalparticles, and ll being rfo ed of the coating metal, and tinuously andsequentially on portions of the continuously subjecting the rolledsubstrate to an elevated temperamoving t i ture at which thesubstantially continuous layer of 11. The method of claim 10 wherein theferrous metal coating metal adheres to the substrate but less than stripis initially cold reduced unannealed blackplate strip, the melting pointof the coating metal until a coand subsequent to rolling the particulatemetal coating the herent and adherent substantially continuous layer ofllnannealed bla kplate strip having the substantially conthe coatingmetal is produced, tinuous layer of coating metal thereon is passedthrough The method of Claim 1 wherein the coating metal acontinuousannealing line to thereby continuously anneal comprises at least onemetal selected from the group conthe blaFkplate strlP and Produce atlghtly adherent layer sisting of aluminum, chromium, copper, nickel andzinc. o of coatmg metal- 3. The method of claim 1 wherein the coatingmetal is The method of clalm 1 Wherem the coatmg P 15 aluminum.alurtnifipm, and the ferrous mlftal subsgat: thavtmdg tthe par 1c a e auminum coa ing ereon 1s ea rea e a a method of clalm 1 wherem h paradesof the temperature of about 400-600" F. in an oxidizing atmoscoatmgmetal have a mean mass particle size of about Phere containing elementaloxygen to agglomerate the 1040' mlcronsaluminum particles and produce athin layer of light The method of dam 1 Wherelh the coatlhg metal 15colored iron oxide on the surface of the substrate under aluminum, andthe particles of the coating metal have a the agglomerated l i i 1 meanmass particle size greater than 15 microns but not 13, Th th d f l i 12h i th b t t i more than 30 microns. heat treated at a temperature ofabout 400-600 F. after rolling to condition the compacted aluminumcoating, and thereafter the substrate is heat treated at a temperatureof at least 800 F. to produce a tightly adherent layer of aluminum.

1-4. The method of claim 12 wherein the substrate having the particulatealuminum coating thereon is rolled at a temperature of at least 400 F.to prevent blistering of the resulting substantially continuous layer ofaluminum.

15. The method of claim 12 wherein the substrate is rolled at atemperature of about 400-600 F., the substrate having the compactedaluminum coating thereon is conditioned by heat treating it at atemperature of about 400 600 F. after rolling, and then the substrate isheat treated at a temperature of about 8501050 F. to tightly adhere thesubstantially continuous layer of aluminum to the substrate.

16. The method of claim 12 wherein the ferrous metal substrate havingthe particulate aluminum coating thereon is cold reduced unannealedblackplate, and subsequent to rolling the particulate aluminum coatingthe blackplate having the substantially continuous layer of --aluminumthereon is heat treated at an elevated annealing temperature to annealthe blackplate and produce a tightly adherent layer of aluminum.

17. The method of claim 12 wherein the substrate is frerous metal strip,the strip is passed continuously through an electrostatic depositionzone and the particulate aluminum coating is electrostatically depositedthereon, the coated stripis passed through a heat treating zone to heattreat the strip at the said temperature of about 400600 F., and the heattreated strip is rolled under pressure, the foregoing steps ofelectrostatic deposition, heat treating at 400-600 F. and rolling beingperformed continuously and sequentially on portions of the continuouslymoving strip.

18. The method of claim 17 wherein the ferrous metal strip is coldreduced unannealed black-plate strip, and subsequent to rolling theparticulate aluminum coating the unannealed blackplate strip having thesubstantially continuous layer of aluminum thereon is passed through acontinuous annealing line to thereby continuously anneal the blackplatestrip and produce a tightly adherent layer of aluminum.

References Cited UNITED STATES PATENTS 1,197,695" 9/1916 Watkins 117-22X1,922,254 8/1933 McCulloch 117-22 X 2,990,293 6/1961 Toulmin 117-93.4 X3,019,126 1/1962 Bartholomew 117-17 3,197,324 7/1965 Brooks 117-213,248,253 4/1966 Bardford et a1. 117-17 3,323,933 6/1967 Bardford et a1.l17-17 3,327,948 6/1967 Gignoux 11793.4 X 3,336,903 8/1967 Point 11717 X3,382,085 5/1968 Wren et al.- 11793.4X

WILLIAM D. MARTIN, Primary Examiner EDWARD J. CABIC, Assistant ExaminerUS. (:1. X.R. 117 22, 31, 50, 65.2, 131

